Coated articles including low-E coatings supported by glass substrates are known in the art. For example, see U.S. Pat. Nos. 6,723,211, 6,686,050, 6,749,941, 6,916,408, and 6,887,575, the disclosures of which are all hereby incorporated herein by reference. Each of these patents discloses a low-E coating supported by a glass substrate. The glass substrate is often a soda-lime-silica type of glass substrate including at least SiO2, CaO and Na2O.
Such coated articles often need to be heat treated. Examples of heat treatment (HT) include thermal tempering, heat bending (e.g., in vehicle windshield applications) and heat strengthening. For instance, in certain applications tempering is required by code (e.g., e.g., for building windows over doorways, for windows identified as breakable windows for firemen, and other applications). Heat treatment (e.g., tempering, heat bending, etc.) typically involves heating the glass substrate with a coating thereon in a furnace at a temperature of at least about 580 degrees C., more preferably at least about 600 degrees C., and often at least about 620 or 640 degrees C. (e.g., for at least about 2 minutes, more preferably for at least about 5 minutes). Thus, it will be appreciated that such heat treatments typically involve very high temperatures.
Unfortunately, due to the high temperatures involved, heat treatment such as tempering and/or heat bending can cause damage to low-E coatings supported by the glass substrate. For example, the high temperatures used in such heat treatments often cause sodium (Na) to migrate from the glass into the coating. If sodium reaches the coating (e.g., the IR reflecting layer(s) that is typically formed of or based on Ag), the coating may be damaged. For example, sodium migration from the glass substrate into the coating can cause the coated article to have undesirable haze, undesirable color, and/or reduced visible transmission following heat treatment.
Thus, it will be appreciated that there exists a need in the art to provide a coated article which is resistant to sodium migration during optional heat treatment.